It is important to study the fast imaging technique of an ultrasound diagnostic system. Implementation of the fast imaging lays a solid foundation for realizing some more advanced technologies. One of the advantages relates to a higher data rate. Larger amount of information obtained in a unit time guarantees a better and more accurate image analysis, thus enabling better executions of various techniques. In particular, the fast imaging technique plays a critical role in the following respects:
1. 3D/4D Imaging
A huge number of data is necessary for both 3D imaging and 4D imaging. Limitation in the imaging speed causes unfavorable effect to the 3D imaging.
2. Blood Flow Imaging
Like image quality, the frame rate of the blood flow imaging has a direct influence on the performance of an ultrasound imaging system, and is an important parameter for evaluating an ultrasound imaging system. Most of the mid-end and low-end ultrasound imaging systems have a relatively low frame rate of blood flow imaging, and can not be compared to the C mode frame rate of a high-end imaging system. Therefore, the ultrasound fast imaging is of great importance. In short, the principle of the ultrasound fast imaging is that data representing a plurality of scan lines are formed in response to one transmit beam. In other words, it is possible to realize a parallel acquisition of scan line data. As a result, the frame rate of the blood flow imaging of the ultrasound imaging system is significantly enhanced.
3. Heart Imaging
For the heart beating with relatively fast speed, the frame rate of an ultrasound imaging system is sometimes more important than the image quality.
4. Image Quality
The existing ultrasound imaging technologies are confronted with a problem of how to balance the image quality and the frame rate. For example:
i) Two beam transmissions can be used to form one scan line with high signal-to-noise ratio (SNR) in the synthetic aperture technique;
ii) The beams transmitted from different directions in different time are used for form scan lines to reduce speckles and increasing imaging quality in the complex imaging technique;
iii) With a coded excitation, the Golay code is transmitted for multiple times to minimize the influence of vertical side lobes;
iv) The high frame rate can be achieved with a low density scan in the B-mode heart imaging.
Imaging quality is improved at the expense of the frame rate in the items i)˜iii), while the high frame rate sacrifices image quality in the item iv). There is a conflict between the frame rate and the image quality. With the ultrasound fast imaging, this conflict can be solved.
5. Heart Related Imaging Technologies
Many existing high-end ultrasound imaging systems relate to heart clinical technologies, such as the anatomical M-mode and cardiac motion analysis. All of them carry out clinical evaluations and index calculations based on the position variation of a certain part of a heart with time so that consecutive images and precise results can be obtained. Therefore, strict requirements are imposed on the temporal resolution of a heart image, i.e., the frame rate of the image.
To improve the frame rate, researchers start to focus on the multi-beam receive technique. In the multi-beam reception technique, multiple receive scan lines are formed in response to one beam transmission, and time for generating a frame of image is thus reduced and the frame rate is increased. In addition to the transmission of a fat beam, another technical challenge confronting the multi-beam reception technique is how to efficiently eliminate distortion of the receive scan lines, which is caused because the receive scan lines are located asymmetrically with respect to the sound field.
The U.S. Pat. No. 6,666,823 B2, entitled “Beam combination method and system”, discloses a multi-beam receiving method, which superimposes the receive scan lines formed respectively in response to two adjacent transmissions to eliminate distortion of the receive scan lines. As shown in FIG. 1, TY1, TY2 and TY3 represent respectively three transmissions, four receive beams along four receive lines are received in response to one transmission. Two of the four receive beams in response to TY1 transmission and two of the four receive beams in response to TY2 transmission are overlapping, and the distortion may be removed by weighting and summing two receive beams along the same receive line.
However, although the above mentioned existing technology may correct distortion, it is realized at the cost of a reduced lateral resolution, because a fat beam has to be transmitted in order to balance energy, which causes the lateral resolution of the sound field to be reduced. To compensate for the decrease of resolution, a large receive aperture may be used, which, however, increases hardware cost.